1. Field of the Invention
The present invention relates to a binary-form dither matrix pattern (dither pattern table) for use in converting continuous tone images (natural images, graphic images, colored characters, and others) into binary images—hereinafter this process will be also called “binary-coding”. The invention also relates to a printing method and a printing apparatus using the binary-form dither matrix pattern.
2. Description of the Related Art
When continuous tone images (natural images, graphic images, and colored characters), which are generated or input from input devices such as scanners, are output from output devices such as printers, the multilevel images must be converted into binary images, so that their continuous tone is expressed by ON/OFF of coloring materials (ink, toner, or the like).
Dithering is a well-known method of such conversion (binary-coding). In dithering, the value of one specific pixel (object pixel) of a continuous tone image (multilevel image) is compared with a predetermined threshold to decide the intensity level of the pixel. In accordance with this intensity level, the ON/OFF of the dot corresponding to the object pixel is determined using a binary-coding dither matrix pattern which has been prepared previously.
For example, Japanese Examined Patent Application Publication No. SHO 53-5561 discloses dither patterns that are tiled in such a way that the centroid of an area to which color materials are to be fixed is at an angle of tanθ=⅓ (approximately 18.43°). In this technique, a dither pattern is rotated at an angle other than 45° so as to avoid moire patterns or other artifacts (artificial striped patterns).
In the dither pattern of Japanese Examined Patent Application Publication No. SHO 53-5561, however, a unit pattern, which is generated by tiling the dither pattern, is composed of a small number (10) of pixels, and thus it is difficult to printout a high quality image of a large number of gray levels. More precisely, to handle images with 256 gray levels (8-bit coded image pixels), it is preferable to use a dither pattern which realizes a unit pattern composed of a larger number of pixels.